In an acid sandy loam soil (pH 3.8), field production of N²O was two orders of magnitude higher at thaw in the spring than at any time during the rest of the year. Soil thaw in midwinter did not result in any increase in N²O flux. Soil water content remained at, or above field capacity during measurements; nitrate was added in excess. This effect could be reproduced in the laboratory: thawing soil cores at controlled temperature, nitrate and moisture yielded a large flush of N²O compared to an unfrozen control. The results indicate the importance of microbial N²O production during thaw for total annual N²O-emission.